Advice needed!!!???

Can anyone give me some advice? I have been giving my baby a bottle of formula before bed for about a week and then continued with breast milk for the rest of he feeds. The last few days he has been really struggling to poo he only managed to have a bowl movement once yesterday and it was very runny but you can see how uncomfortable it is making him. He has also come up with a rash which sometimes looks worse than other times. I have also been prescribed some antibiotics for my self which I started taking on Thursday. I was told this would not affect breast feeding.

Does anyone have any advice on what I could do to help my baby? Or what they think the reason for this could be? Thank you! X